The invention relates to a circuit arrangement for igniting and supplying a lamp with a DC current, comprising
input terminals for connection to terminals of a power supply source supplying a DC voltage, PA1 a DC/DC converter coupled to the input terminals and provided with PA1 an inductive element, PA1 a first unidirectional element, PA1 a first switching element, PA1 a control circuit coupled to a control electrode of the first switching element for rendering the first switching element conducting and non-conducting at a frequency f, PA1 output terminals for connecting the lamp, and PA1 an output capacitance shunting the lamp during operation, and comprising PA1 a first branch connecting the output terminals during operation and comprising a second switching element and means for rendering the second switching element conducting and non-conducting. PA1 a light-transmissive discharge vessel having a filling of rare gas, and two electrodes, PA1 a lamp housing secured to the discharge vessel, PA1 a lamp base provided with electric contacts and secured to the lamp housing, and PA1 a ballast circuit coupled between the contacts and the electrodes for generating a lamp current from a power supply voltage.
The invention also relates to a compact lamp.
A circuit arrangement as described in the opening paragraph is known from U.S. Pat. No. 5,581,161. In the known circuit arrangement, the DC/DC converter is a down-converter. Immediately after the known circuit arrangement is put into operation, both the first and the second switching element are conducting. During operation of the known circuit arrangement, both a first and a second lamp electrode form part of the first branch. The inductive element and the first branch convey a current during a first time interval, so that the electrodes of the lamp connected to the circuit arrangement are preheated. At the end of the first time interval, the second switching elements rendered non-conducting so that the first branch no longer conveys any current. The inductive element subsequently charges the output capacitance via the first unidirectional element, which output capacitance supplies the lamp with an ignition voltage. After igniting the lamp, during stationary (i.e. stable) operation, the control circuit renders the first switching element high-frequency conducting and non-conducting, and a lamp connected to the circuit arrangement is fed with a DC current supplied by the down-converter. Since the down-converter consists of only a small number of components, the known circuit arrangement can be manufactured relatively easily and is thus inexpensive. A drawback of the known circuit arrangement is, however, that the ignition voltage generated at the end of the first time interval has only a relatively small amplitude so that the lamp does not easily ignite. In the known circuit arrangement, the second switching element is constituted by a glow discharge starter. If the lamp does not ignite immediately, the glow discharge starter will successively become conducting and non-conducting a number of times, while an ignition voltage is generated across the output capacitance every time after the glow discharge starter has become non-conducting. It has been found in practice that, dependent on the specific properties of the relevant lamp, the ignition of the lamp is only successful after a number of attempts or, in the worst case, fails completely.